High density polyethylene (HDPE) is a low-cost, high performance thermoplastic material. In its crystalline state, HDPE has superior physical properties and good thermal stability. Being a thermoplastic, it can be economically processed by an injection molding machine.
HDPE is presently the plastic material of choice for making automobile fuel tanks. It has excellent tensile and impact properties at temperatures as low as -50.degree. C. and at temperatures as high as 70.degree. C. which is the temperature range normally experienced by a fuel tank in service. Since HDPE has low cost, it is competitive with steel which is presently used for making most automobile fuel tanks.
One drawback of HDPE that needs improvement in its continued use for plastic fuel tanks, is the permeation of HDPE to automotive fuels. At present, this drawback is being compensated by treating the interior surfaces of the HDPE tank with chlorinating or sulfonating agents. These treatments are effective against gasoline permeation but are less effective in gasoline/alcohol blends. Another problem is that the chemical used in the fluorinating or sulfonating treatments are toxic and corrosive and pose some potential health hazards.
One other approach to improve the permeability of HDPE is to blend it with highly crystalline polar materials. One such candidate is polyethylene terephthalate (PET). Polyethylene terephthalate is a low-cost material and an excellent barrier to non-polar fuel components. However, it has low impact strength and a low melt viscosity which are inadequate for fuel tank applications. Furthermore, blends of high density polyethylene and polyethylene terephthalates are highly incompatible due to their large differences in polarity and melt viscosities which result in materials having poor physical properties as compared to either component of the blend. A method for compatibilizing the blend is therefore needed before the blend can be effectively used in making automobile fuel tanks. Some work was performed to compatibilize high density polyethylene/polyethylene terephthalate blends by using styrene-butadiene-styrene block copolymers or acrylic acid grafted polypropylene with very limited success. Only minor improvements in tensile strength, flexural strength and impact strengths were noted.
It is, therefore, an object of the present invention to provide a polyblend of high density polyethylene/polyethylene terephthalate having improved impact properties.
It is another object of the present invention to provide a polyblend of high density polyethylene/polyethylene terephthalate having low permeation properties to gasoline fuels such that the polyblend can be used in molding automobile fuel tanks.
It is a further object of the present invention to provide an moldable, high impact strength polyblend of high density polyethylene/polyethylene terephthalate for use in automobile fuel tanks applications.
It is yet another object of the present invention to provide a low cost, moldable polyblend of high density polyethylene/polyethylene terephthalate having superior impact properties and low permeation properties to gasoline fuel such that it can be used in molding automobile fuel tanks.